It is known to manufacture the joists and other supporting structures in buildings in the form of light concrete beams arranged adjacent one another and each one consisting of an elongate concrete coffer closed by a concrete slab with some sound-insulating material inserted in the gap between the slab and the coffer. One has found that sound-insulating means consisting of equidistantly positioned blocks of rubber or other suitable resilient material arranged in gap between the slab and the coffer give a satisfactory sound-insulating effect while at the same time offering the possibility of providing an air passage between neighbouring beams.